The present invention relates to a liquid crystal device for use in flat-panel displays, projection displays, printers, etc., and a liquid crystal apparatus including the liquid crystal device.
As a type of nematic liquid crystal display device used heretofore, there has been known an active matrix-type liquid crystal device wherein each pixel is provided with an active element (e.g., a thin film transistor (TFT)).
As a nematic liquid crystal material used for such an active matrix-type liquid crystal device using a TFT, there has been presently widely used a twisted nematic (TN) liquid crystal as disclosed by M. Schadt and W. Helfrich, Appl. Phys. Lett., vol. 18, no. 4, pp. 127-128 (1971).
In recent years, there has been proposed an In-Plain Switching mode of liquid crystal device utilizing an electric field applied in a longitudinal direction of the device, thus improving a viewing angle characteristic which is problematic in TN-mode liquid crystal displays. Further, a liquid crystal device of a super twisted nematic (STN) mode without using the active element (TFT. etc.) has also been known as a representative example of the nematic liquid crystal display device.
Accordingly, the nematic liquid crystal display device includes various display or drive modes. In any mode, however, the resultant nematic liquid crystal display device has encountered a problematic slow response speed of several ten milliseconds or above.
In order to solve the above-mentioned difficulties of the conventional types of nematic liquid crystal devices, a liquid crystal device using a liquid crystal exhibiting bistability (xe2x80x9cSSFLCxe2x80x9d, Surface Stabilized FLC) has been proposed by Clark and Lagerwall (Japanese Laid-Open Patent Application (JP-A) 56-107216, U.S. Pat. No. 4,367,924). As the liquid crystal exhibiting bistability, a chiral smectic liquid crystal or a ferroelectric liquid crystal (FLC) having chiral smectic C phase (SmC*) is generally used. Such a chiral smectic (ferroelectric) liquid crystal has a very quick response speed, because it causes inversion switching of liquid crystal molecules by the action of an applied electric field on spontaneous polarizations of their liquid crystal molecules. Accordingly, the chiral smectic liquid crystal is considered suitable for constituting a display device or a light valve of a high speed. Further, the liquid crystal device using the chiral smectic liquid crystal generally effects light transmission by utilizing birefringence to provide a bright state, thus realizing a relatively wide viewing angle characteristic.
In recent years, as another liquid crystal material, an anti ferroelectric liquid crystal showing tristability (tristable states) has caught attention. Similarly to the ferroelectric liquid crystal, the anti ferroelectric liquid crystal causes molecular inversion switching based on the action of an applied electric field on its spontaneous polarization, thus providing very highspeed responsiveness. This type of liquid crystal material has a molecular alignment (orientation) structure wherein liquid crystal molecules cancel or counterbalance each others"" spontaneous polarizations under no electric field application, thus having no spontaneous polarization in the absence of the electric field.
The above-mentioned ferroelectric and anti ferroelectric liquid crystal causing inversion switching based on spontaneous polarization are liquid crystal materials assuming smectic phase (chiral smectic liquid crystals). Accordingly, by using these liquid crystal materials capable of solving the problem of the conventional nematic liquid crystal materials in terms of response speed, it has been expected to realize a smectic liquid crystal display device.
As described above, the (anti-)ferroelectric (or chiral smectic) liquid crystal having a spontaneous polarization has been expected to be suitable for use in displays exhibiting highspeed response performance in the near future.
In the case of the above-mentioned device (cell) using the (anti-)ferroelectric liquid crystal exhibiting bistability or tristability, however, it has been difficult to effect a gradation display in each pixel due to its display principle.
In recent years, in order to allow a mode of controlling various gradation levels, there have been proposed liquid crystal devices using a specific chiral smectic liquid crystal, such as a ferroelectric liquid crystal of a short pitch-type, a polymer-stabilized ferroelectric liquid crystal or an anti-ferroelectric liquid crystal showing no threshold (voltage) value. However, these devices have not been put into practical use sufficiently.
Our research group has proposed a liquid crystal device using a chiral smectic liquid crystal having a phase transition series on temperature decrease of Iso (isotropic phase)xe2x88x92Ch (cholesteric phase)xe2x88x92SmC* (chiral smectic C phase) or Isoxe2x88x92SmC* (U.S. patent application Ser. No. 09/338,426 filed Jun. 23, 1999). The chiral smectic liquid crystal device has provided practical advantages in terms of highspeed responsiveness, massproductivity, etc.
Specifically, when the chiral smectic liquid crystal having the phase transition series of Isoxe2x88x92Chxe2x88x92SmC* or Chxe2x88x92SmC* on temperature decrease is disposed in a uniaxial aligned cell, liquid crystal molecules are oriented or aligned to provide a smectic layer normal direction (a direction of a normal to smectic molecular layers) deviating from the uniaxial aligning treatment (axis) direction. By controlling the deviating layer normal direction of smectic liquid crystal molecules in one direction, the liquid crystal molecules are aligned in a direction such that a molecular position corresponding to only one of two stable states in SmC* present in parallel with substrates substantially coincides with the uniaxial aligning treatment (axis) direction, thus monostabilizing the liquid crystal molecules in one direction. As a result, the liquid crystal molecules can assume an intermediate light-transmission state depending on a voltage applied thereto while retaining a highspeed responsiveness.
Such a liquid crystal device, however, provides different display characteristics such that a relatively bright display state is obtained depending on an applied voltage of one polarity and a relatively dark display state is obtained under application of an applied voltage of the other polarity when driven using an AC driving waveform. As a result, even in the case of setting a frame frequency of an input picture (image) signal to 60 Hz, an actual frame frequency for gradational display becomes 30 Hz, thus causing a remarkable flickering.
Further, the liquid crystal device has a poor viewing angle characteristic such that a white color tone changes from a desired level when viewed from a side or oblique direction. This is attributable to different retardations (optical path difference) between a retardation in a long molecular axis direction and a retardation in a short molecular axis direction when the liquid crystal device is observed from its oblique direction, since a bright state is given by light transmission based on the birefringence effect. When a viewing angle is closer to the long axis direction of liquid crystal molecules, a resultant refractive index anisotropy (birefringence) is liable to change compared with the case where the viewing angle is in other directions, thus being liable to change or invert a gradational level and color tone in the long molecular axis direction. On the other hand, in the short molecular axis direction perpendicular to the long molecular axis direction, the resultant refractive index anisotropy is not changed, but the optical path is increased with a tilting (inclination) of the viewing angle. As a result, white color tone is shifted toward a yellowish tint in the short molecular axis direction.
In view of the above-mentioned problems, an object of the present invention is to provide a liquid crystal device and a liquid crystal apparatus including the liquid crystal device capable of effecting clear gradational image display with a high gradation reproducibility while suppressing an occurrence of flickering.
Another object of the present invention is to provide a liquid crystal device and a liquid crystal apparatus using the liquid crystal device capable of suppressing a change in color tone when viewed from its oblique direction to improve a viewing angle-dependent color tone characteristic while retaining a clear motion picture display performance.
According to the present invention, there is provided a liquid crystal device comprising a chiral smectic liquid crystal, a pair of substrates disposed to sandwich the liquid crystal and having thereon electrodes for applying a voltage to the liquid crystal so as to form a plurality of pixels, each provided with an active element connected to an associated electrode on at least one of the substrate wherein the liquid crystal is aligned to form domains D1 and D2 having mutually different directions of normal to smectic layers, and the liquid crystal has an alignment characteristic in each of the domains D1 and D2 such that the liquid crystal is aligned to provide an average molecular axis to be placed in a monostable alignment state under no voltage application, is tilted from the monostable alignment state in one direction when supplied with a voltage of a first polarity at a tilting angle which varies depending on the magnitude of the supplied voltage, and is tilted from the monostable alignment state in the other direction when supplied with a voltage of a second polarity opposite to the first polarity at a tilting angle, said tilting angles providing maximum tilting angles xcex21 and xcex22 formed under application of the voltages of the first and second polarities, respectively, satisfying:
xcex21 greater than xcex22 greater than 0 in domain D1,
and
0 less than xcex21 less than xcex22 in domain D2.
According to the present invention, there is also provided a liquid crystal display apparatus comprising the liquid crystal device and drive means for driving the active elements of the liquid crystal device.
These and other objects, features and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.